The present invention relates generally to reflector antennas, and more specifically to reflector antennas operating in multiple frequency bands.
Antennas with paraboloidal reflectors are commonly used for satellite communications in which radio frequency signals are typically transmitted between an earth station and a satellite, or vice versa. Paraboloidal reflector antennas are also used in radar and other communications applications as well. Such antennas are typically constructed in a prime focus configuration where microwave frequency energy is coupled to a transceiver by an antenna feed mounted near a focal point of the main paraboloidal reflector. Other commonly used antenna configurations include Gregorian and Cassegrain which employ a small ellipsoidal or hyperboloidal subreflector mounted near the focal point of the main paraboloidal reflector. A Gregorian or Cassegrain antenna typically includes a feed located between the main reflector and the subreflector.
The purpose of an antenna feed is to connect a transceiver to the reflector. Antennas intended for operation over multiple frequency bands normally require a corresponding number of multiple feeds and subreflectors. As a result, antenna construction and operation may become quite complicated as a result of the differences in the wavelengths among the different frequency bands and the associated physical structure of the antenna. Antennas have typically been designed for transmissions in both the C and KU-bands. The C-band covers frequencies from about 3.6 GHz to 6.5 GHz. The KU-band covers frequencies from about 10.9 GHz to 14.5 GHz. The wavelengths between these two frequency bands can vary from about 3 inches for the C-band down to about 1 inch for the KU-band. More recently, antennas have been required to handle satellite communications in the X-band covering frequencies from about 7.2 GHz to 8.4 GHz. The wave guiding and wave handling structures of the antenna must be physically matched to the length of the electromagnetic waves being handled. The need to receive and transmit signals in the different bands from a single antenna dish system has created several problems. The different geometries required for handling electromagnetic waves in several different bands (e.g. C, KU and X-bands) has caused significant difficulties in receiving and processing both frequency bands. In addition, mutual blockage between antenna feeds typically occurs due to the use of several different feeds in the same antenna configuration.
Several different devices have been used to resolve the difficulties associated with processing multiple frequencies. For example, as described in Varley, R. F., "EHF Satcom Terminal Antennas", Session Record 3, Southcon 1982, Electronic Conventions, Inc., El Segundo, Calif., a dual reflector antenna in a Cassegrainian configuration with a dichroic subreflector has been used to reduce blockage between two different feeds. Similarly, a coaxial feed, such as disclosed in U.S. Pat. No. 5,636,944 to Weinstein et al., allows for simultaneous transmission and reception in the C-band and either the X or KU-bands. However, none of the known prior art antenna structures provide an antenna structure that successfully allows for the simultaneous transmission or reception of waves in the C, X and KU bands with significant reduction in mutual blockage.
In addition to difficulties with construction and arrangement, current multiple frequency antenna systems also suffer from aberration problems. An antenna beam may suffer from some sort of aberration if its feed is located away from the geometrical focus thereby preventing the production of a radiated planar wavefront. These aberration problems may be corrected through the use of an array feed system. However, known multiple frequency antenna systems do not include an array feed system and an aberration correction capability.
Polarization refers to the direction and behavior of the vector associated with the electric field of the electromagnetic signal which is radiating through free space (i.e. empty space with no electrons, ions or other objects to distort the radiation). In signals with linear polarization, the electric field vectors sinusoidally reverse their direction in a plane which is orthogonal to the radiation path, but they do not rotate. If the orientation of the vectors is vertical, the signal is said to have vertical polarization; if the orientation is horizontal, the signal is said to have horizontal polarization.
In contrast, if the direction of the electric filed vectors rotates at some constant angular velocity then the signal is said to have elliptical polarization. Signals with elliptical polarization can be effectively generated by combining two linearly polarized signals which are oriented in a orthogonal relationship and which have a predetermined phase difference between their electric field vectors. Circular polarization is a special case of elliptical polarization in which the two linearly polarized signals have electric field vectors of equal magnitude and a phase difference of 90 degrees. Satellite communications are typically conducted with circularly-polarized signals because of the resistance of the signal to multipath distortion, but are unable to achieve polarization purity due to cross polarization.
The cross polarized component in the antenna beam is the orthogonally polarized (e.g. vertically polarized versus horizontally polarized or right hand circularly polarized versus left hand circularly polarized) signal unintentionally present with the intended (i.e. co-polarized) component of polarization. A signal that includes the unintended component is typically referred to as lacking polarization purity. The cross polarized component has the effect of reducing the signal strength in the co-polarized component and increasing interference with signals of the orthogonal polarization. A receiver that includes polarization diversity is capable of handling two orthogonal polarizations independently and purely.
As discussed above, current multi-feed antenna systems have many shortcomings and it is an object of the present invention to obviate many of these shortcomings and to provide a novel multiple feed antenna system and method.
It is another object of the present invention to provide a novel reflector antenna and method that is capable of transmitting and receiving simultaneously in multiple frequency bands.
It is still another object of the present invention to provide a novel reflector antenna and method for minimizing mutual blockage between antenna feeds.
It is a further object of the present invention to provide a novel reflector antenna and method to provide full polarization of diverse elements to correct for cross polarizing components and achieve polarization purity.
It is yet another object of the present invention to provide a novel high efficiency reflector antenna and method with correct phase and amplitude illumination of the reflector.
It is yet a further object of the present invention to provide a novel reflector antenna and method of fully polarizing diverse elements to accommodate polarized signals of any sense and orientation.
It is still a further object of the present invention to provide a novel reflector antenna and method having a flexible design for multi-purpose applications.
It is still another object of the present invention to provide a novel reflector antenna and method utilizing fixed phase and amplitude weights to provide a low cost design for steady state operation.
It is yet another object of the present invention to provide a novel reflector antenna and method utilizing variable phase and amplitude weights for adaptive optics that address temporal changes.
These and many other objects and advantages of the present invention will be readily apparent to one skilled in the art to which the invention pertains from a perusal of the claims, the appended drawings, and the following detailed description of the preferred embodiments.